1. Field of the Invention
The present invention relates to obtaining and processing of seismic data. More particularly, the present invention relates to determining possible continuity of subsurface earth formations or beds between existing wells, for reservoir evaluation purposes and the like.
2. Description of the Prior Art
It has long been desirable to analyze and evaluate structure and stratigraphy of petroleum producing formations. One purpose, for example, has been in conjunction with secondary and tertiary recovery operations. In recent years, there has been increased interest in seismic methods for reservoir definition and description. One method has been the crosswell seismic method where seismic energy emitted from sources in one well or borehole was sensed and recorded as seismic data by arrays of detectors in one or more other wells. The recorded seismic data were then processed to form tomographic images of interwell geologic features based on the crosswell seismic data.
Present techniques using crosswell seismic data have been based on tomographic reconstruction for interwell seismic velocity imaging. The tomographic imaging technique used for geophysical applications has been adapted from that used extensively in medical imaging.
When used for geophysical purposes, tomographic imaging between boreholes could produce very good images provided certain conditions were present. Unfortunately, in most petroleum reservoirs, these conditions have been quite difficult to achieve.
Since the imaging solutions were intended to produce two-dimensional results, the well boreholes had to be aligned; that is, the boreholes had to be vertical or both had to deviate in the same plane.
Another problem was that a large data sample or window of observed transmission energy was required. To obtain such data, the seismic source was required to emit energy in the source well to sensors in the receiver well or wells at depths of several hundred feet both above and below the target reservoir or formation of interest. Since it was rare for wells to be drilled deeper than the formations of interest, this condition was very hard to achieve.
The tomographic reconstruction or imaging process was based on an interwell velocity model. Such a model required precise positioning or location data for the well, in addition to the observed signal travel times. A final condition was accurate location data for the wells.